1. Field of the Invention
This invention relates generally to electro-optical measurement of physical properties and more particularly to a method and apparatus for self-diagnosis and automatic compensation of the entire electro-optical instruments particularly adapted for continuous in situ measurement properties of materials, such as for example, the mass per unit volume of particulates or aerosols entrained in a gaseous medium.
The invention is particularly adapted for use in connection with electro-optical methods and systems for making in situ particle measurements of the type described in U.S. Pat. Nos. 3,797,937 and 4,017,186 and the subject matter of the latter patents is hereby incorporated by reference. Reference is also made to the Instructional Manual of the Model C20 Particulate Monitor of PPM Inc. which illustrates and describes an embodiment of the invention in connection with the continuous in situ measurement of oil particulates entrained in a cryogenic process helium flow. The subject matter of the latter Instruction Manual is hereby incorporated by reference.
2. Description of the Prior Art
One of the oldest and most widely used electro-optical devices is the optical transmissometer. In recent years, its principal application has been monitoring the opacity of stack emissions. A well-known example is monitoring the effluent of electric generating facilities wherein particulates resulting from combustion of coal or oil are emitted up the stack and into the ambient environment. Under restricted conditions, the opacity, or better the optical density, may be roughly correlated with the mass concentration of the particulates in the stack gases. Thus, higher opacity or optical density indicates larger emission levels. Currently, in many countries, including the U.S., regulations are written to limit either (or both) the opacity or the mass emission rates.
Transmissometers operate by transmitting a more or less collimated beam of electromagnetic radiation (usually visible or "photoptic" light) across the stack and measuring the degree of attenuation thereof. In some instruments, the beam is reflected by a mirror system on the opposite side of the stack from the source, back into the source unit, thus permitting a double pass and, more practically, permitting incorporation of both source and receiver beams in the same package. Some instruments permit automatic compensation of the apparatus, but not of the entire electro-optics train and not without interrupting data acquisition.
There are several thousand transmissometers currently in use for opacity monitoring. Clearly, the quality of and the confidence in the measurements could be greatly enhanced if means were available to readily trace the calibration of the instruments to a central, industry-accepted, facility. Still further, superior measurements not now possible would be realized if data were not lost during the calibration cycle.
Another form of electro-optical instrument useful for measuring particulate concentrations is a nephelometer. U.S. Pat. No. 3,563,661 to Charlson and Alhquist describes an integrating nephelometer, which also is a light scattering device. This apparatus measures primarily the scattering coefficient which is related to optical visibility effects in the earth's atmosphere. Visibility reductions result from increasing aerosol concentrations in the atmosphere and are very sensitive to the aerosol particle size, in fact, more so than to the mass concentration of the particles. These devices are calibrated by simulation in the laboratory with known aerosols or by Rayleigh scattering from known gases. Such devices have found limited application because they require relatively sophisticated and highly trained personnel for operation. The calibration obviously requires interruption of data collection and may not be directly traced, being on an individual instrument basis.
In U.S. Pat. No. 3,782,824 to Stoliar and Brown, there is disclosed an apparatus and method for measuring the extinction coefficient of an atmospheric scattering medium using the so-called LIDAR or laser backscattering technique. Again, this electro-optical method yields a property of the earth's atmosphere relating to visibility effects. Whereas the aforementioned integrating nephelometer of Charlson and Alhquist measures light scatter integrated over essentially the zero to 180.degree. angular range and within a small length of space internal to the instrument, the backscatter technique of Stoliar and Brown collects backscattered light within a very small angular range near 180.degree., over a large region (length along beam) of space. The amplitude of the backscattered yield is related through well-known equations in the art to the properties of aerosols illuminated by the pulsed laser beam. However, the general use of the technique is on a relative indicating basis and absolute calibration is virtually impossible. As with the integrating nephelometer, some attempts have been made to correlate backscattering yield with mass concentration of the aerosols. In certain circumstances this correlation is a fairly satisfactory indicator, but the calibration is not traceable and does not automatically compensate the measurement apparatus.
The aforementioned U.S. Pat. No. 3,797,937 of Shofner describes a system for making particle measurements by forward scattering of laser light. Specifically, the measurements are for the size distribution of relatively large droplets and the apparatus employs an external in situ, sampling volume. No provision is made for either automatic or traceable calibration.
The more recent U.S. Pat. No. 4,017,188 of Shofner and Kreikebaum, describes an electro-optical method and system for in situ measurements of particulate mass density using backscattered LED light. This patent discloses that, under certain circumstances, the backscattered signal correlates well with mass concentration (not mass density as specified in the title) of particulates entrained in a gaseous medium. This patent also discloses a means by which this particular system may be calibrated relatively in the field by insertion of a "standard scattering medium" into the beam. Calibration is achieved by interruption of the measurement process through use of a remotely positioned scattering plate which is brought into position after the sensing head is withdrawn particularly from its housing. No provision is made for automatic calibration/zero compensation or for providing traceable calibration information.